At 42 Avenue Foch, the tree-lined boulevard that is one of Paris's most expensive streets, looms a five-storey private mansion complete with disco, spa room, hair salon, gold- and jewel-encrusted taps, lift, pastel pink dining room and a breathtaking balcony-view of the Arc de Triomphe.

Local people always knew when there was about to be a visit from its 41-year-old "playboy" resident, Teodorin Obiang, eldest son of the autocratic president of Equatorial Guinea. Days before Obiang Jr's private jet touched down, two massive lorries would pull up outside and disgorge a sea of fresh flowers to dress the interior of the mansion.

When Obiang was in residence, passersby would see a parade of couturiers from Paris's top design houses, including Yves Saint Laurent, Dior, Louis Vuitton, waiting to be admitted for fittings before returning with vanloads of made-to-measure clothes. Crates of the most expensive burgundy were another regular delivery.

On one occasion 15,000 DVDs were hauled in on wooden pallets – roughly 41 years worth of viewing.

But the most public statement of opulence was the fleet of luxury, turbo-charged, yellow, red and blue sports cars, parked in garages or in the cobbled courtyard.

"The noise-factor was extreme," one local said. "He seemed obsessed with security so when he wanted to go out between midnight and 2am, he'd order the chauffeur to warm up four cars so no one knew which he'd take. Can you imagine the noise of Ferraris, Porsches and Maseratis all running at once? Then he'd come down and decide to take a fifth car and that would have to be started."

But the courtyard has fallen quiet, the mansion empty of occupants. Three months ago, in a morning raid, French police towed away 11 luxury cars, including a Maserati, a Porsche Carrera, an Aston Martin and a Mercedes Maybach.

At the time, Obiang Jr, whose salary as Equatorial Guinea's agriculture and forestry minister was €3,200 (£2,700) a month, owned two Bugatti Veyrons, the most expensive and fastest street car in the world, costing about €1m a piece and reaching 250mph . He was in the process of acquiring a third.

The raid was the first in the landmark French inquiry known as the case of the "ill-gotten gains".

In an unprecedented move, three serving African leaders and their families are under investigation in Paris over whether they embezzled state funds to acquire vast assets in France including bank accounts, Riviera villas and fleets of luxury cars.

The clan of Gabon's late leader Omar Bongo and its current leader, his son Ali Bongo; the Congo-Brazzaville leader, Denis Sassou-Nguesso and his family, and President Teodoro Obiang of Equatorial Guinea and his clan are accused of having assets worth €160m in France, from penthouses and villas to scores of bank accounts and luxury car fleets.

The leaders and their families have denied building up personal wealth in France through embezzlement, money-laundering and misuse of public funds.

Judges are beginning the detailed task of trying to prove such spectacular wealth was directly siphoned from the coffers of the oil-rich states to the detriment of populations left to live in misery.

In 2000, just as Obiang began building up his car collection, Equatorial Guinea was on paper the wealthiest African country per inhabitant, yet a majority of its people lived below the UN poverty threshold.

With billions of dollars worth of assets of Muammar Gaddafi frozen by the UN and member countries, and other legal moves to recover the wealth of deposed autocrats such as Tunisia's Zine al-Abidine Ben Ali and Egypt's Hosni Mubarak, the drive to seize billions plundered by corrupt leaders has never been higher.

But the French case against three serving African leaders, initiated by anti-corruption NGOs as part of a long legal battle, also illustrates the limits on western willingness to act against rulers still in power.

"With deposed heads of state after the Arab spring, there was no problem, the whole community was scandalised at the plundering of money from their countries. We're warning against double standards: why should you have to wait for a leader to fall to put a stop to corruption?" said Maud Perdriel-Vaissiere, head of Sherpa, one NGO leading the case.

The police inquiry has given an unprecedented insight into the lifestyle of certain African leaders. When the spectacular art collection from the homes of the late fashion designer Yves Saint Laurent went up for auction in Paris in 2009 it was called the art sale of the century and raised more than €370m. French authorities later revealed that Obiang Jr bought 109 lots at the sale, costing €18m.

Olivier Pardo, lawyer for Equatorial Guinea in France, said the case of the "ill-gotten gains" violated international law and that he would contest the case and pursue France through the international courts. It may not just be France.

A US government court action is seeking to seize $71m (£45m) of assets from Obiang Jr in the US which it claims were paid for through corruption. His US assets are said to include a $38.5m Gulfstream V private jet, a $30m mansion in Malibu, California, and $1.8m of Michael Jackson memorabilia, including a white crystal-covered glove and a crystal-covered pair of socks. A spokesman for Equatorial Guinea denied wrongdoing. An inquiry into the Obiangs' assets is also underway in Spain.

On a corner of Paris's chic Avenue Rapp, in the heart of the wealthy 7th arrondissement, a short walk from the Eiffel tower, the gentlemen's outfitters Pape tells its own story of the lifestyle of leaders of oil-rich states.

The Senegalese tailor Pape Ibrahima N'diaye, a Paris institution known as "Monsieur Pape", is a favourite of French lawyers, politicians and businessmen.

Denis Sassou N'Guesso, the 68-year-old leader of Congo-Brazzaville, famous for his impeccable suits and dress sense, did not hold back in his private fittings.

A new book, The Scandal of the Ill-Gotten Gains, by the investigative journalists Thomas Hofnung and Xavier Harel, has sent shockwaves through the French establishment with fresh details of spending habits.

In it, the authors reveal a note by Tracfin, the French anti-money laundering authority, which states that in April 2010, Sassou N'Guesso ordered 91 suits from Pape for €276,000. A month earlier, in March 2010, he had bought 48 shirts for €24,000. In one year, in the 12 months from November 2009, Sassou N'Guesso spent more than €652,000 on clothes there. His lawyer dismissed the sum as "false and absurd".

The Sassou N'Guesso clan have 24 properties in France in their own name, 112 bank accounts and various sports cars. Meanwhile, NGOs point out that 70% of Congo-Brazzaville people live on less than $1 a day.

In the heart of the 8th arrondissement, not far from the French president's Elysée palace, a mansion on the quiet Rue de la Baume has come to symbolise the wealth of Gabon's late leader Omar Bongo. When Bongo died in 2009, he was the world's longest-ruling head of state, save for the British and Thai monarchies.

A friend of all recent French presidents, at one time he owned more Paris properties than any other foreign leader. The Bongo clan has the biggest property portfolio in the "ill-gotten gains case".

A preliminary police report claimed he and his close relatives own 39 properties in France, mostly in exclusive districts of Paris and on the French Riviera. They also have 70 French bank accounts and at least nine luxury cars in France, including Ferraris and Mercedes worth a total of €1.5m. Bongo's son, Ali Bongo, was elected president in 2009 after his father's death. That year he bought himself a Bentley Continental Flying Spur for more than €200,000, which can run for 1,500 miles without refuelling despite the fact that oil-rich Gabon has less than 500 miles of asphalt roads.

Anti-corruption campaigners have already trooped past the €18.9m mansion on Rue de La Baume, bought in 2007 in the name of two Bongo children, then 13 and 16, and other relatives, in what some call Paris's "ill-gotten gains" walking tour. Rooms are full of unopened boxes of electrical kitchen goods which suggest the house hasn't yet been moved into. A broken doorbell hangs from a wire beside the entrance.

When in power, Omar Bongo's official salary was reportedly €14,940 to €20,000 a month. French officials are now investigating where the money came from. A 2007 French police report indicated that the payments for some vehicles "appeared at least atypical".

The case has shed further light on allegations of French collusion in corruption under the cosy system known as "françafrique" – in which kickbacks, petrodollars and privileged relations defined Paris's foreign policy towards its former colonies.

Paris, with its image as the capital of the art of living, was always popular with high-spending dictators. ¬

The question now is what impact the case has in other countries where serving leaders placed their cash.

Tim Daniel, a British lawyer and anti-corruption specialist, said: "London is a No 1 destination for kleptocrats." But British law does not allow for cases to be brought by NGOs, so any recovery of assets would depend on the money-laundering authorities.

One of Nigeria's most influential and wealthy politicians, James Ibori, has been charged in London with 25 offences relating to alleged money-laundering and fraud and will soon face trial.

"The international anti-corruption community is following the French case intensely," Daniel said. "It's a very interesting precedent."